Apparently it is also a good idea to put the weaker hive on the top part of the split. I have heard that during cooler months, the heat generated from the bottom colony rises and helps to insure that the struggling bees up top will have an easier time overwintering.

Don't know if that's just conjecture or not (haven't tried it myself), but it would make sense.

In cool weather I just try to keep a split in the right size for the number of bees. I do two frame mediums for my mating nucs and they are a kind of split. I put in a frame of brood and a frame of honey and a queen cell. I only tried a nuc on top of a strong hive for overwintering and the moisture was a big problem. It wouldn't be as much of a problem in the spring as in the winter, I would guess.